In recent years, the information amount has drastically increased by spreading of high definition video distribution, and improvement in the information processing capability of a data center or the like is demanded. Further, it is demanded to implement improvement of the information processing capacity by a low-cost and low-power consumption device, and, in recent years, silicon photonics is investigated lively.
For example, it has been proposed to vary the dimensions in both of a thicknesswise direction and a widthwise direction of a waveguide core stepwise in order to couple different devices with low loss. This is referred to as first technology.
Also it has been proposed to achieve spot size conversion with low loss, for example, by forming a waveguide core such that the width of an upper waveguide layer formed at a central position in a widthwise direction on a lower waveguide layer is reduced to allow an optical distribution to transit from the upper waveguide layer to the lower waveguide layer. This is referred to as second technology.
Further, since the cross sectional shape of a silicon waveguide core used for such silicon photonics as described above is so small as, for example, approximately 500 nm in width and approximately 220 nm in height, a mismatch with the spot size (for example, approximately several μm to 10 μm) of an optical fiber occurs. This gives rise to excessive coupling loss.
Therefore, a spot size converter has been proposed in which, in order to suppress excessive coupling loss, the width of the silicon waveguide core is reduced in a taper shape and the silicon waveguide core is covered with a second core to cause light to transit from the silicon waveguide core to the second core thereby to increase the spot size. This is referred to as second core type spot size converter.